1. Field
The present invention relates generally to a method of manufacturing thin-film photovoltaic devices, and particularly to a method for the manufacturing of high-efficiency thin-film photovoltaic devices.
2. Related Art
Photovoltaic (PV) devices represent one of the major sources of environmentally clean and renewable energy. They are frequently used to convert optical energy into electrical energy. Typically, a PV device is made of one or more semiconducting materials with p-doped and n-doped regions. The conversion efficiency of solar power into electricity of this device is limited to an absolute theoretical maximum of about 37%, since photon energy in excess of the semiconductor's bandgap is wasted as heat. In practice, PV devices with a single p-n junction may achieve a maximum efficiency of 25%. Currently, efficiencies of the best commercially available PV cells do not exceed 22%.
A number of different photovoltaic (PV) technologies have been developed in the last several decades. Most of these technologies may be divided into two large groups: one that is based on crystalline semiconductors and the other one that is based on polycrystalline and amorphous materials. The first group is largely based on semiconductors, such as silicon (Si), germanium (Ge) and gallium-arsenide (GaAs) that are grown on special, high quality, expensive substrates. The second group has a more diverse material base, which includes different forms of Si, Ge and their alloys, compound semiconductors such as cadmium-telluride (CdTe), copper-indium-selenium (CIS) and copper-indium-gallium-selenium (CIGS), organic molecular and polymeric semiconductors and others. These materials are typically deposited as thin films with thickness in the range of 1-10 μm, using a variety of methods and low-cost substrates. Thus, this latter group of PV technologies is often referred to as thin-film technologies.
The main attraction of thin-film PV is its relatively low cost of manufacturing. However, thin-film PV devices are not as efficient as crystalline PV devices. For example, average efficiencies of a-Si, CdTe, and CIGS PV modules are about 10% or less. On the other hand, the average efficiencies of c-Si modules are greater than 15%. In spite of relatively lower efficiency, thin-film PV technologies are capable of achieving lower cost per Watt (power in Watts produced by a PV cell) and therefore much shorter payback time. As a result, such technologies are gaining popularity and represent the fastest growing segment in the current PV market. Strong efforts are now under way to improve the performance of thin film PV devices in order to achieve efficiencies close to those demonstrated by crystalline PV devices.